Deflection limits to beam sizes and support moments 1

[IJR]

My dear pals

In most codes ACI inclusive, there is a relation between Mu and bw * d2 where a limit is set for Mu to make sure that serviceability conditions (deflection often cited) are satisfied and designer has to go ahead with ultimate state limit design

Are these only for span moments or do they cover support moments as well.

No one clarified that yet. At supports we dont have deflections, do we?

my respects
ijr

 

[UcfSE]

I'm not sure what you mean about that for the ACI. The ACI has tables in chapter 9 that tell you minimum depths to provide for which deflections need not be computed for normal loading. At any rate since there is no deflection at a support I wouldn't think you would use a support moment to find deflection.

 

[IJR]

UcfSe

Thanks for just challenging me- I can see the light now and probably have found the answer. Here, to share with you

The limit for tensile reinforcement in a beam is that corresponding to 0.85*balanced steel, right? this keeps beams ductile.

so you proportion the beam for Mu such that your steel in the tension zone never exceed that much.

If it does exceed you have an option to use compression steel and ensure that total rho-rho,compression does not exceed 0.85*balanced steel.

And an option to change size.

0.85*balances steel does keep the beam ductile. Compression steel also does keep the beam ductile

We havent talked about deflection yet. Only ductility. Which is applicable in both support and span sections

Now lets talk about deflections.

1) You use the guides you have mentioned-the L/D ratios

2) What if you dont satisfy and you can not satisfy those L/D ratios?. You have to check deflections-of course at span location, not for span moments. And that does it.

3)But you know that deflection is related to amount of tension steel. The serviceability level calculations take advantage of that steel because the modular ratio is high (7-8). And So some codes like the one I am using tend to limit steel ratio well below the 0.85*balanced steel, aiming at forcing you to do calculation checks and provide better depth.

No matter what, deflection considerations do not apply to supports, and my post is void.

Thanks for joining in.

regs
IJR

 

[UcfSE]

That sounds interesting. What code is that? If I remember correctly our code, the ACI 318, used to limit tensile steel to 0.75 rho_balanced. It has removed that requirement in favor of requiring a minimum tensile strain of 0.004. The minimum tensile strain to use a phi of 0.9 is 0.005. Between 0.004 and 0.005 you have to interpolate phi based on steel strain. The limit of 0.004 comes out to being close to the former limit of 0.75 rho_b. I think it's come out to about 0.71 rho_b or so.

Typically when I have concrete beams I have masonry above them. Because masonry has stricter deflection requirements I usually have to check the deflection for the beam the long way instead of simply being able to look in a table and provide a minimum L/D. To keep deflections from being excessive I typically put a limit of 0.1% on rho, from a rule of thumb we use in the office.

It's interesting to hear what others must do when using a different code than the one with which I am familiar.